1. Field of the Invention
The invention relates to actuators, such as Lorentz actuators, and also to velocity transducers.
The invention also relates to a lithographic projection apparatus, and more particularly to a lithographic projection apparatus that has a Lorentz actuator connected to a mask table or a substrate table of the lithographic projection apparatus.
2. Discussion of Related Art
Lorentz actuators comprise a permanent magnet, which produces a magnetic field, and a current element positioned in the magnetic field. They work on the same principle as an electric motor, namely that charge carriers moving through a magnetic field experience a force mutually perpendicular to their velocity and the magnetic field, known as the Lorentz force. The force is given by Jxc3x97B, where J is the current vector resulting from the velocity of the charge carriers and B is the magnetic field vector. This Lorentz force is used to induce motion or to provide a bias force between the moving parts of the actuator.
Lithographic projection apparatuses can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the mask (reticle) may contain a circuit pattern corresponding to an individual layer of the IC, and this pattern can then be imaged onto a target area (die) on a substrate (silicon wafer) which has been coated with a layer of photosensitive material (resist). In general, a single wafer will contain a whole network of adjacent dies which are successively irradiated through the reticle, one at a time. In one type of lithographic projection apparatus, each die is irradiated by exposing the entire reticle pattern onto the die in one go; such an apparatus is commonly referred to as a waferstepper. In an alternative apparatus, each die is irradiated by progressively scanning the projection beam over the reticle pattern, and thus scanning a corresponding image onto the die; such an apparatus is referred to as a step-and-scan apparatus. Both of these types of apparatus require highly accurate relative positioning of the mask and substrate tables, which is generally accomplished with the aid of at least one Lorentz actuator. More information with regard to these devices can be gleaned from International Patent Application WO 97/33204.
Up to very recently, apparatuses of this type contained a single mask table and a single substrate table. However, machines are now becoming available in which there are at least two independently movable substrate tables; see, for example, the multi-stage apparatus described in International Patent Applications WO 98/28665 and WO 98/40791. The basic operating principle behind such multi-stage apparatuses is that, while a first substrate table is underneath the projection system so as to allow exposure of a first substrate located on that table, a second substrate table can run to a loading position, discharge an exposed substrate, pick up a new substrate, perform some initial alignment measurements on the new substrate, and then stand by to transfer this new substrate to the exposure position underneath the projection system as soon as exposure of the first substrate is completed, whence the cycle repeats itself; in this manner, it is possible to achieve a substantially increased machine throughput, which in turn improves the cost of ownership of the machine
In currently available lithographic devices, the employed radiation is generally ultra-violet (UV) light, which can be derived from an excimer laser or mercury lamp, for example; many such devices use UV light having a wavelength of 365 nm or 248 nm. However, the rapidly developing electronics industry continually demands lithographic devices which can achieve ever-higher resolutions, and this is forcing the industry toward even shorter-wavelength radiation, particularly UV light with a wavelength of 193 nm or 157 nm. Beyond this point there are several possible scenarios, including the use of extreme UV light (EUV: wavelengthxcx9c50 nm and less, e.g. 13.4 nm or 11 nm), X-rays, ion beams or electron beams.
One problem with Lorentz actuators is that, when no current flows, there is no force between the moving parts. When a current is caused to flow to overcome this, it results in dissipation of heat in the device. This is particularly a problem in applications which require the actuator to deliver a bias force, e.g. to support the weight of a component under gravity. With this continuous need to compensate for weight, a base power dissipation is unavoidable, and can cause problems with heat sensitive apparatuses, such as optical devices which require accurate alignment; on the other hand, it necessitates the provision of additional cooling power.
Another problem is that, when such actuators support a load in order to act as isolation bearings, the stiffness of the bearing should be low so as to avoid the transmission of vibrations. Conventionally, it has been difficult to provide such low-stiffness isolation bearings.
Velocity transducers can also operate on the Lorentz principle, by virtue of the fact that the motion of a component through a magnetic field induces a current flow or a resulting EMF which can be measured. In order to measure velocities down to very low frequencies, it is necessary to have a transducer with a very low frequency of resonance, which conventionally has been difficult to achieve. This is because of the problems in producing a transducer with a very low stiffness.
It is an object of the present invention to alleviate, at least partially, some of the above problems.
Accordingly, the present invention provides a device comprising:
a first member comprising at least one main magnet, and
a second member comprising at least one current element for carrying an electric current, for electromagnetic interaction with said main magnet,
characterized in that said second member further comprises an auxiliary magnetic member which interacts with the magnetic field of said main magnet to produce a bias force between said first and second members.
The invention also relates to a lithographic projection apparatus comprising a radiation system for supplying a projection beam of radiation; a mask table provided with a mask holder for holding a mask; a substrate table provided with a substrate holder for holding a substrate; a projection system for imaging an irradiated portion of the mask onto a target portion of the substrate; and further comprising a Lorentz actuator connected to at least one of the mask table and the substrate table.
The auxiliary magnetic member can be a permanent magnet. Alternatively, it can comprise a ferromagnetic material (e.g. a soft-iron member). In this latter case, as long as the stroke of movement of the current element/auxiliary magnetic member is relatively small (as will generally be the case in applications in short-stroke lithography actuators, for example)xe2x80x94such that the auxiliary magnetic member remains biased to one side of the centerline of the whole assemblyxe2x80x94magnetic fluxes going through the ferromagnetic material of the auxiliary magnetic member will produce a bias force component in the desired direction; while less than that produced in the case of a permanent magnetic material, this force will be quite sufficient for particular applications.
The device according to the invention can be substantially planar or cylindrical, and the main magnet can be magnetized perpendicular or parallel to the bias force.
Preferably, the device further comprises a third member, also comprising at least one further main magnet.
The current element may be a coil, and the auxiliary magnetic member is preferably located at a plane substantially centrally between two halves of the coil.
Advantageously, the effective stiffness of the device is 200 N/m or less in magnitude, and ideally close to zero.
The device can be used as an actuator and/or a velocity transducer.
The device can have two second members stiffly connected to each other and arranged such that opposite parasitic torques are generated in each second member, which thereby cancel out.
Advantageously, the actuator and/or transducer of the present invention can be used in lithographic projection apparatuses. A great advantage of the invention in such an application is that it provides a bias force capable of counteracting, for example, the weight of the table (chuck) in a wafer stage or reticle stage, and yet does so without the heat dissipation associated with current flow, thus helping to maintain a well-defined and constant local temperature. This is important, since the nanometer-accuracy commonly required of such apparatuses can only be satisfactorily achieved in a highly controlled environment, wherein unnecessary sources of heat and/or contamination (e.g. as a result of evaporation or outgassing) are highly undesirable. Such considerations are of particular importance in a vacuum environment, in which context it should be noted that lithographic apparatus for use with radiation types such as EUV, electron beams, ion beams, 157-nm UV, 126-nm UV, etc. will most probably comprise a vacuum along at least part of the radiation path within the apparatus.
In a manufacturing process using a lithographic projection apparatus according to the invention, a pattern in a mask is imaged onto a substrate which is at least partially covered by a layer of energy-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book xe2x80x9cMicrochip Fabrication: A Practical Guide to Semiconductor Processingxe2x80x9d, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4.
Although specific reference has been made hereabove to the use of the apparatus according to the invention in the manufacture of ICs, it should be explicitly understood that such an apparatus has many other possible applications. For example, it may be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal display panels, thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms xe2x80x9creticlexe2x80x9d, xe2x80x9cwaferxe2x80x9d or xe2x80x9cdiexe2x80x9d in this text should be considered as being replaced by the more general terms xe2x80x9cmaskxe2x80x9d, xe2x80x9csubstratexe2x80x9d and xe2x80x9ctarget areaxe2x80x9d, respectively.